NTA. The grandparents should celebrate their anniversary the day before or after. Or if it HAS to be on that day, they can stop by at the party and leave early to celebrate their anniversary. They’ve been married for how long? They can’t let go of a date to make their grandson happy? I agree with OP. They are grown and should have the capability to be empathetic towards their 7 year old grandkid. Move the anniversary date or don’t come to any of my kids’ birthdays. It’s their job to make it work if they want to be a part of their grandchildren’s lives. The kid only has so many years during his childhood. He will remember the unequal treatment when he grows up. The grandparents have had many years to celebrate already, I don’t see the big deal in moving their anniversary celebration to another date/time in order to accommodate for their grandson.

TLDR: grandma needs to grow up